Facsimile system



June 23, 1942. E. BRUCE EIAL 2,287,413-

FACSIMILE SYSTEM Filed Oct. 31, 1940 4 She ets-Sheet 1 RUCE AT RNEV 2 LINE ADVANCE lL/NE ADVANCE .55 INVENTORSWHERR/OTT June 23, 1942. E. BRUCE ETAL 2,287,413

FACSIMI'LE SYSTEM v Filed Oct. 31, 1940 4 Sheets-Sheet 2 FIG. 2

I .E. BRUCE INVENTORSWHERR/OTT ATTORNE Y 1942- E. BRUCE ETAL 2,237,413

FACSIMILE SYSTEM Filed Oct. 51, 1940 4 Sheets-Sheet 3 FIG. 3

A T Tom/Ev June 23, 194.2. E. BRUE ET AL mcsmnim' sysmm Fil'ed Oct. 51, 1940 4 Sheets-Sheet 4 I .5. BRUCE INVENTORS'WHERR/OTT A TTORNE V Patented June 23, 1942- FAosIMrLE SYSTEM Edmond Bruce, Red Bank, and William Herriott,

Ohatham, N. 1., assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a

corporation of New York Application-October 31, 1940, Serial No. 363,598

19 Claims. (Cl. 178-65) This invention relates to signaling systems and particularly to a system for deriving image signals from fields of view, especially graphic material such as written or printed matter, documents and the like, transmitting the same to a receiver and there utilizing them for reconstitution. A principal object of the invention is to increase the speed of transmission and so reduce the time required.

' A related object is to omit, as far as possible, the time-consuming transmission of signals derived from the blank portions of the field of view which contain no information as to its substantive content but only as to its form.

In pursuance of these and other objects of the invention, two independent scanning spots are caused to traverse successive elemental areas of the field of view to be transmitted, which for the sake of illustration may be taken as consisting of dark typewritten matter on a white paper backgorund, 'to give rise simultaneously to two separatev and independent scanning signals. A scanning aperture associated with one of these scanningspots is separate from andindependent derived from this advance or premonitory aperture may be utilized to cause the apparatus as a whole to skip a blank portion of the copy, that is, a portion which is devoid of dark elemental areas throughout a, stipulated length. For ex- 9 ample, if the spots are separated by the width of sponding means for advancing an impressionreceiving element at the receiver. The circuit arrangement and biases are such that when the of another scanning aperture associated with the I other scanning spot. Likewise, the associated translating devices, for example, photoelectric cells, are separate and independent. The independent scanning spots are preferably spaced apart on the copy to be scanned, for example in a direction perpendicular to the line scanning direction and by the Width cf a single scanning I independent electric signals are derived. Signals thus derived from one of the scanning spots, which may be termed the main scanning spot, constitute theusual image signals which may be transmitted to a receiver for reconstitution in the usual way. The aperture associated with this 'spot may be termed the main scanning aperture. scanning spot, which is the first to traverse any particular portion of the copy and may therefore be termed the advance spot by vintue of its dis- Other signals are derived from the other position; or, by virtue of its function, the auxiliaryjfor premonitory spot; while the sperture associated therewith may be similarly designated. In accordance withthe invention, signals auxiliary aperture, in the course of scanning a stipulated part of the copy, for example. a full scanning line, has seen one or more dark elemental picture areas, the selector causes the first copy-advancing means at the transmitter to operate to advance the copy by the width of a single line. When, on the other hand, the auxiliary aperture has seen no dark areas at all in the a course of scanning a full line, the selector governs the operation of the second copy-advancing means to advance the copy by the width of two lines instead of one. In each case corresponding single-line or double-line advance of the impression-receiving element at the receiver is eflected under control of the transmitter apparatus.

In accordance with one embodiment of the invention, a band of the original copy, at least as broad as two full scanning lines, is strongly illuminated. Light from two separate elemental areas of this illuminated band, spaced apart by the width of a scanning line, is gathered through an optical system and projected into two separate photoelectric cells, being focused on two aper tures placed in front of the cells. The two areasfrom which the light'is thus gathered are swept side by side together over the copy in the course 'of the scanning operation. Thus, though other areas of the band may be illuminated with equal intensity, the particular areas that are momentarily in conjugate relation with the apertures serve as the scanning spots. I

In accordance with a modification of the optical arrangements, two small spots of light may be projected from two apertures, iluminated by a lamp or lamps behind them, onto the plane of the copy to illuminate two separate elemental areas of the copy, spaced apart preferably by the width 01? one scanning line. Light reflected from the corresponding different color sensitivities, either by the use of diiferent photoelectric materials or by the use of filters in front of the cells.

The invention will be fully understood from time the photoelectric cells are preferably given the following detailed description of an illustrative embodiment thereof taken in connection with the appended drawings in which:

Fig. 1-is'a diagram, partly inperspective, show- I ing apparatus in accordance with the invention;

Fig. 2 is a diagram in perspective of a modification of a part of the apparatus of Fi 1;

Fig. 3 is a diagram of a modification of other parts of the apparatus of Fig. 1; and

Fig. 4 shows receiver apparatus in accordance with the invention.

The embodiment of the ensuing detailed de- Y scription and drawing constitutes a modification of the apparatus described and shown in copending application Serial No. 336,538, filed May 22, 1940, now Patent 2,262,584 issued Nov. ,11, 1941. This, however, is for purposes of illustration only, since the invention is equally applicable to systems of other types, for example, systems in which scanning is accomplished by the continuous rotation of a cylinder around which the copy is wrapped, or in which scanning is accomplished by a tape having perforations uniformly spaced along its length as shown, for example, in H. J. Nichols Patent 2,176,680, October 1'1, 1939.

Referring now to Fig. l, a sheet of copy I may be advanced by a supporting conveyor, for example, an endless'belt 2 which passes around a driving'roller 3 and a driven roller 4 which are arranged to turn on shafts 5 and 8 in journals, not shown, mounted on a framework or base I of the apparatus. upper or working part of the belt may be secured by an idler roller 8 which may conveniently be swung on brackets pivoted at the journal points Correct tension and flatness of the of one of the rollers, for example, the driven roller 4. Brackets 9 mounted on the framework or base I support two guide plates, an upper one fixed to the brackets with its lower face in the plane of the upper surface of the upper part of the belt and bearing lightly against it, and a lower one l2 which is yieldably mounted on springs l3 to bear against the underside of the same part of the belt, and'press it lightly and snugly against the upper guide plate I I, being re strained from moving with the belt by pins ll extending through slots IS in the brackets 9. The lower surface of the fixed guide plate II is preferably finished by polishing or otherwise to a fine, smooth, fiatsurface.

It should be understood that terms such as "upper and "lower which may be employed in this speciiication with reference to the figures and as indicating parts of the apparatus are employed merely to avoid circumlocution, since the apparatus or any part thereof may of course be mounted in any desired orientation.

The belt 2 preferably comprises a fabric base to prevent excessive stretching and is preferably provided with a resilient yielding upper surface of substantial thickness, for example, of rubber.

. The belt 2 is arranged to be intermittently advanced between the guides |l,. l2 in short steps by engagement of either of the relay operated pawls 2|, 22 with the teeth of a ratchet wheel 28 which drives the driving roller 3 through a worm 24 and gear 25. The pawls 2|, 22 are held in disengaged position by springs 28, 21 and are arranged to engage the teeth of the ratchet and advance the latter when their respective relays 28, 29 are energized. The extent of the movement of each pawl is adjusted by correct placement of an associated stop 30, 3|. These stops are preferably so placed that actuation of one of the pawls 22 advances the ratchet wheel 23 by the length .ofone tooth and actuation of the other pawl 2| advances the ratchet wheel 23 by the length of two teeth. The associated gear ratio is soselected that advance by the distance of one tooth corresponds to advance of the copy I by the width of a single scanning line while double tooth advance corresponds to advance of the copy by a double line width.

A crank 32 may be mounted on the drive roller shaft 5 for rapid manual advancementof the belt, to permit which, a freewheeling device, for example, a ratchet and pawl 33, is interposed between the worm gear." and the roller shaft 5.

The upper guide plate I I is pierced by a slot 40 extending transversely of the belt 2 slightly less than the full width of the guide plate H and over a distance at least as great as'the widest copy to be scanned. In the drawing, thewidth of the slot has been greatly exaggerated in order to allow space to show separation of light beams in accordance with the invention. The width of this slot need not in fact be greater than the width of a few full scanning lines, for example, a few hundredths of an inch.

Long filament lamps 4| are mounted close to. this slot and preferably on each side of it in position to illuminate as brightly as possible a band of the copy where it is exposed'in the slot 40. One lamp will serve, but stronger illumination is secured with two placed as shown. These lamps may be provided with a reflector or a condensing lens system, for example, a cylindrical lens, in order that the illumination of the exposed band of copy may be as intense as possible. Furthermore, theupper edges of the slot may be widely beveled to admit light from these long filamer t lamps'through wide angles and to permit light reflected from the copy to 'emerge'unimpeded; and the lower edges of the slot are preferably slightly rounded to prevent their catching the copy and scratching or tearing it.

The leading end or the guide plate u may be provided with a sloping riser to facilitate admission of the copy between it and the belt.

The guide plate may, of course, be of transparent material/for example plate glass, or it may be provided with a glass insert'instead of a slot. However, on account of the fact that glass becomes scratched, the open slot above described is preferred.

Mounted in cooperative disposition with that portion of the copy which is exposed through the slot is an optical system. comprising" a rocking mirror 42, a reciprocating lens 0, a fixed lens 44, a fixed mirror ll, and a shield l0 containing two small apertures ll, 0. these apertures defines or limitsthe size and shape of animage of "asingle elemental area of the copy; With optical system whose Each of T in the copy plane;

' magnification is unity, .and scanning lines spaced 100 to the inch, each ofthese apertures may be 0.01 inch square. If the magnification of the optical system is other than unity, the size of the apertures may be altered accordingly.

The mirrors and lenses are disposed to bring two elemental areas of the copy I, spaced apart by the width of a single scanning line, into conjugate focal relation with the apertures 41, 48. This condition is indicated on thedrawing by the dotted lines 58, each of which represents the path of a single ray of the light beam joining an elemental copy area to its aperture.

The fixed mirror 45 is a convenience and permits greater compactness of the apparatus, but it is in no sense essential.

The rocking mirror 42 is preferably mounted vertically above the mid-point of the scanning slot 40 in a frame which may be pivoted to rock on journals 52 about an axis lying in the mirror face and fixed with respect to the apparatus framework. As shown the mirror 42 is arranged to be rocked by reciprocation of links 53, 54 by rotation of a cam 55 which is slidabiy mounted on a shaft 56 arranged to be driven through change gears 51 by a motor 58. Rocking vof this mirror causes different elemental areas of the copy to be imaged on the apertures 41, 48 in succession. In so doing, however, thetotal path length from the copy I to the apertures 41, 48 is altered so that focal points which are'optically conjugate to the apertures no longer lie To maintain the elemental copy areas always in conjugate focal relation with the apertures, provision is made to .reciprocate the lens 43 in just the proper amount to compensate for errors in focus caused by the motion of the rocking mirror 42. sion, which is fully described in the aforementioned Patent 2,262,584 and forms a part of the subject-matter thereof, is accomplished by the use of another cam 59 which, being mounted on the same. shaft 56 as the mirror cam, rotates synchronously therewith. A

In the course of the advanceof the belt 2 each elemental area of the copy first meets the scanning line in which the light beam 58 which is directed toward the aperture 41 originates.

This aperture maytherefore be termed the ad- ,ning beam 5| and therefore to the light values of successive elemental areas scanned is supplied to a direct current amplifier 65 to provide image signals. These image signals may then be transmitted directly to the receiver but it is preferred to transmit instead a high frequency carrier modulated in accordance with the picture signals. Therefore there is provided a -2000-cycle oscillator 66 whose output energy is supplied, together with that from the direct current amplifier 65 to a modulator 61 and thence to the line 88 and output terminals 69.

The light of the beam 50 which passes the aperture 57 is reflected by a prism 62 and procurrent flows through a resistor Ill. The result- .1 By .virtue of the characteristics of this discharge tube, the anode current continues to flow ing voltage drop across this resistor is impressed on the input circuit of a suitable trigger device, for example, a gas discharge tube II which, in addition to the usual cathode l2 and anode 18, contains a starting anodeor grid 14. This gas discharge tube is biased, for example by adjustment of the voltage of a bias bat- .tery I5, to remain below its cut-off point as long as full photocell current, corresponding, to bright portions of the copy I, flows through the resistor I0. At the same time its bias is such that when the premonitory light beam 50 is momentarily reduced in strength, as for example, when a 'dark elemental area of the copy is imaged on the premonitory aperture 41, ignition of the tube takes place and anode current from a battery 16 or other suitable source flows in the cathode-anode circuit of the tube.

thereafter until stopped by other means; in other words, the tarting anode or grid I4 of the tube loses control so that subsequent increase' in the strength of the premonitory-beam This provi- 50, due for example to imaging light elemental copy areas on the premonitory aperture, has

no effect.

The anode I3 of the gas discharge tube H is connected in series with alrelay coil 11 and .a switch 18 which is normally closed but is arranged to be opened momentarily once in the course of eachrevolution of a cam 18 which .is fixedly mounted on the drive shaft 56. The armature 80 of the relay I1 is held by a spring,

in position to close a contact 8| in the absence of relay current and the spring tension is so adjusted that when current-,fiows through the relay coil 11 the armature 88 isurged against 4' the spring tension and the contact 8| is opened and another contact 82 is closed instead. The relay armature 80 itself is connected through a battery 83 or other current source, through a switch 84 to ground. The switch, 84, which is normally open, is arranged to be momentarily closed once in the course of eachrevolution of the drive shaft 56 by the cam 19. The contact 8| is connected in series with relay coil 28, re-

jected onto a photoelectric cell 64- whose output sistor and battery 86 to ground and the contact 82 is similarly connected in series with relay coil 29, resistor 81 and battery 86 to' ground, so that closure of the switch 84 energizes one or other of the relays 28 or 29, to advance the copy by a double line width or a single line width respectively, depending on whether or not the relay -'I1 is energized by -anode current of the discharge tube 1|.

The switches 84 and 18 are so placed with respect to each other. and the knob 19a of the cam 19 is so placed with respect to the operative faces of the cams 55 and 59 that the switches 84 and 18 are operated in succession and during the fiyback time of the mirror 42. For example, the switch 84 may be closed immediately I after the completion of a line scan .and the switch 18 opened just prior to the commencement of the next line scan.

To provide suitable signals for advancing the recorder apparatus at the receiver station by a single line width or a double line width as required in step withthe advance of the copy at the transmitter, there may be provided two separate signal sources of readily distinguishable signals, one or other of which momentarily sup plies energyto the transmission channel leading .to the receiver station at the conclusion of each line scanI Thus, for example, a generator 90 of oscillations of a frequency different from the image signal carrier frequency, for example, 1000 cycles per second, is connected to the input circuit of an amplifier tube 9|, whose output current, when the amplifier is rendered operative, flows through a resistor 92 giving rise to a corresponding signal on the line 68 across which it is connected, and a second oscillator 93 which generates oscillations of a still difierent frequency, for example 500 cycles per second, is similarly connected to the input circuit of an amplifier tube 94 whose output current, when the tube is energized, flows through a resistor 95 giving rise to a corresponding signal on the line 68 across which it is connected. The amplifiers 9|, 94, normally biased below cut-ofi by the voltages of batteries 86, 88, are rendered operative in the alternative by removing their steady biases and placing on them instead a more positive bias of the battery 83 through one or other of the relays 28, 29, each time the cam-operated switch 84 is closed.

The operation of the apparatus as described I above is as follows: Assuming that the premonitory aperture 41 has seen no black elemental areas-during a full line scan, then, as above, explained, the auxiliary discharge tube II will not be ignited, the selector relay 11 will not be energized andits armature 80 will remain in the position shown under tension of its spring so that the selector contact 8| remains closed and the contact 82 remains open. Under these conditions, at the conclusion of the full line scan the cam-19 momentarily closes the battery switch 84. This serves to apply the voltage of the battery 83 to the double line advance relay 28 and at the same time to remove the negative bias of the amplifier 9|. The relay 28 is energized and advances the transmitter copy I by the width of the two lines and at the same time, the amplifier 9| being rendered conductive, a 1000-cycle signal from the oscillator 90 is placed on the line 68 for A transmission to a receiver where it may operate appropriate mechanism for advancing the imaccount of the fact that the selector contact 82 is open.

use at the conclusion of the main line scan. This stored impression is then erased by opening of. the switch 18. Impression-storage devices of other types may occur to those skilled in the art as being suitable for use in this connection.

At the receiver the 1000-cycle double-line advance signals, the 500-cycle single line advance signals and the image signals may be separated from each other and directed into the devices they are intended to operate by suitable filters in, well-known manner, -If for any reason such separation is found undesirable, the various signals' of the invention may, of course, be transmitted over separate channels, by carrier modulation or otherwise.

It will be apparent that the system of the invention permits a substantial increase in the over-all speed of transmission of image signals,

' particularly in the case of printed, typewritten contains entirely blank lines of substantially the same width as the character lines, and doublespaced typed copy contains blanks of still greater width. The apparatus of the invention serves to reduce the transmission time of blank areas to one-half of what it would otherwise be and therefore, with copy of this' sort, effects a considerable reduction in the total transmission time for the copy as a whole.

In the system above described, an area of copy extending the full width of the copy to be Assume in the second place that during the scanned and at least as wide as two scanning lines was strongly illuminated, for example by long filament lamps 4| placed on either side of the scanning slot 40. It will be apparent that the greater part ofthis light is lost, since only two elemental picture areas oi. the band are utilized at one time. If desired, light can be economized by varying the optical arrangement as shown in Fig. 2. Referring to Fig. 2, two separate light sources I00, l0l are provided whose -light is directed by prisms SI, 62 onto two sepconditioned for excitation when in the course I arate scanning apertures 41, 48. The light of the two sources is preferably of different color characteristics, and this is indicated by the provision of a color filter I02 of one color, for example, ared filter, in the light path or the source I00 and a filter I03 of different characteristic, for example, a blue filter, in the light path of the source WI. The filters may be placed at any. convenient'point in the light paths and each of them receiving medium by the width of a single line. then reflected by the-stationary mirror 45 onto the rocking mirror 42 which serves to sweep the images of the illuminated apertures across the copy. These images, which serve as scanning spots, are preferably spaced apart by the width of the scanning line in the manner above described in connection with Fig. 1. "A stationary lens 44 and a reciprocable lens 43 are provided in the manner above described to focus the apertures 41, 48 on the copy and maintain this conjugate focal relationship throughout the sweep of the scanning spots. This arrangement strongsive devices are placed in the vicinityof the.

scanning slot 40 and disposed to receive as much of the reflected light as possible, independent of the position of the scanning beams along the scanning lines. These photocells may, for example, be of the long cathode type described in Patent 2,262,584 and the light received by each cell may be augmented by any appropriate arrangement of lenses or reflectors as likewise disclosed in Patent 2,262,584.' In order to insure that the main photoelectric cell I64 shall respend only to light from the main scanning'beam 5i and that the advance photocell I65 shall likewise respond only to light from the advance scanning beam 56, a further color separation maybe employed. Thus, for example, the main scanning cell I64 may be provided with a redsensitive cathode, for example a caesium compound, and the auxiliary scanning cell I05 may be provided with a blue-sensitive potassium cath ode. If preferred, a red color filter I66 may be disposed in the path'of the light from the copy I to the main scanning photocell I64 and a blue color filter I01 correspondingly disposed in the path of the light from the copy, to the advance scanning photocell I65. This arrangement serves to insure separation of the signals from the principal scanning spot and the advance scanning spot and confinesthem to their respective photoelectric cells and associated circuits.

The associated electrical and mechanical apparatus may be identical with that shown in r Fig. 1 and fully described above.

Referring again to Fig. 1, earns and 59 are arranged for scanning copy of the full width of slot 40 'as above described and for maintaining the effective scanning areas in focus throughout the scanning operation by coordinated movement of the mirror 42 and the lens 43. Provision is made, however, for scanning copy of reduced width and to this end the cams, instead of being fixedly mounted on the shaft 56, are fixedly mounted on a sleeve 56 which is arranged to rotate with the shaft 56 but is free to be moved along the latter in an axial direction. Another pair of cams 55 and 59' are mounted on the 59 into engagement therewith. The cam 55? is preferably cut to retmn the mirror 42 to its initial position after swinging the beams 50 and 5I over one half the length of the slot 40 and the cam 59 is correspondingly cut to preserve sharp focus of the images of the apertures 41 and 48 on the copy I throughout the resulting reduced beam motion. Since, under these conditions, the speeds of the individual moving parts will have been greatly reduced, the speed of the apparatus as a whole may be increased, as by shifting the gears 51, thereby eflfectin'g an increase in the speed of transmission of the picture. These arrangements are in the main the same as those fully described in the aforementioned Patent 2,262,584.

No attempt has been made in the drawings to exhibit the precise form of the cam faces. Rather, the description of the operation and efiect of these cams, for a fuller description of which reference is again made to Patent 2,262,584, should be taken as determining their form.

Fig. 3 indicates schematically'a modification adapted to scanning copy of any arbitrary width equal to or less than the capacity of the appa-' ratus which is limited only by the length of the scanning slot 40. For this arrangement the shaft I56 instead of being rotated continuously, as in Fig. 1, is rotated through-a preassignedangle against the tension of a shaft-returning spring I5! and then, upon the release of magnetic clutch I58, snapped back to its starting point where its motion is arrested by stops I66. The clutch I56 is energized from a source I6i and the electric circuit is completed through a relay I62 and contacts I64. The current through'the relay I62 may be adjusted to hold the contacts I64 closed against the tension of a spring I65 when a current is flowing as by'initial adjustment of the variable resistor I63 connected across it. The scanning mirror I42 is provided with two arms I66, I61 adjustable with respect to eachother,

the angle betweenthem being measurable by a I scale I68 which may be graduated in units of copy width. The arm I6? is disposed to close the contact I64 when the mirror I42 is in position to direct the scanning beams I50, I5I to the beginning of a line of the copy and so. permits the clutch I58 to be energized and scanningto commence. The other arm I66 is disposed to open the contacts I64 when'the mirror I'42 has been swung through an angle preset on the scale I68, corresponding to a sweep of the scanning beam over a full line of copy of the reduced I64 are thus points, and at the sametime the relay I62 is deenergized. The spring I65 holds the contacts I64 open until the return sweep of the scanning beam is complete. Thereupon the arm I61 closes these contacts oncemore to allow the clutch I56 and the relay I62 to be energized and so to commence the scanning of I the following line. this arrangement, the cams I55, I 59 do not rotate continuously in one direction but rather rotate through a certain angle determined by the setting on. the scale I68 and then snap back to their starting points. It is desirable that the line-advancing signal occur after the scanning 'ofany one line is completed and before the scanning of the following'line commences; that is to switch I18 successively during the counter-clockwise fiyback motion, and the tension of the spring I82 is so adjusted that in the course of the clockwise scanning motion, neither of these switches I84, I is affected.

The electrical circuits associated with these switches, including generators, amplifiers, trigger devices, copy-advancing relays, selector relays and all other apparatus may be identical with those above described or may be modified therefrom in various particulars as desired.

Fig. 4 shows one form of receiver apparatus embodying the invention which may, by way of example, be employed for translating the si nalsdelivered by the transmitter apparatus above described into a replica of the original copy. Referrin to this figure, signals are-received at input terminals 200 and are then directed into three separate paths the first of which 201 supplies the reproducer apparatus proper, the second 202 supplies sin le line advance apparatus, and the third 208 supplies double lineadvance apparatus.

1 In the first path the input terminals 200 are connected to a detector 204 whose output termiiginating at the. cathode sharply on the comb 2 I 2, preferably on a single one of the comb conductors.

' The discrete conductors which protrude in connection with Fig. 1

nals are connected to the input terminals of a reproducer device; for example they may be connected by way of a biasing battery 200 to the cathode 208 and control grid 201 of a cathode L ray reproducer tube of the type described and I claimed in copending applications Serial Nos. 328,816 filed April 10, 1940 and 328,819, filed April 10, 1940, now Patents 2,273,433 issued Feb. 1'7, 1942, and 2,273,793, issued Feb. 17, 1942,1'especrtlvely. Briefly} this device comprises an evacu- -;.a.ii d vessel containing an electron emissive cathone 208, afocusing anode 208 containing a beamdefining aperture 209, an accelerating anode 2| 0, 4

'a pair of beam deflecting elements '2, and a conductive comb 2l2 comprising a plurality of discrete conductors arranged in a single row and extending parallel to each other through the end wall of the vessel. The distance from the center line of each conductor of the comb to the center line of an adjacent conductor should preferabiy be not greater than the width of a single elemental area of the copy or field to be reproduced; for example, the distance'may be of the order of 0.01 inch. The beam receiving end of tube may be enlarged in one dimension and flattened out into the form of a blunt wedge'in another dimension and its inner walls are preferably provided with a conductive coating or lining 2l2 which is connected to the acceleratin anode 210 in order to provide an electron collecting surface near the beam receiving comb 2i2 and to eliminate charges on the glass. For the purposes of this application the device is also provided with an electrode or grid 201 for modulating the cathode beam in accordance with the signal strength. Any desired means, however, [may be employed to secure beam modulation. Suitable sources of potential, for example batteries 2, are connected between theelectrodes ito supply operating voltages thereto. Battery voltages and electrode arrangements and spacare selected tofocus an electron beam orbeam will strike it at a greater orless distance from the point at which the conductor passes through the tube wall even though the focusing of the cathode beam in a direction perpendicular to the scanning direction be more or less imperfect. The comb depicted in the drawings follows the improved construction of Patent.

2,273,793 which serves the same purpose as well as others.

In order to' secure a high current output it is preferable that electrons of the beam strike as large an area of each individual comb conductor as possible. To this end the electron beam may be given the cross-section of a narrow rectangle and focused in a narrow rectangular spot on the comb conductor or conductors by the use of the suitably shaped aperture 208 in the focusing anode 208. The same result may also be secured by other means, for example, the means described and claimed in Winans application Serial No. 307,255 filed December 2,1939, now Patent 2,268,195, issued Dec. 30, 1941.

It is of advantage to spread some high resistance fluorescent material such as zinc silicate the cathode m, passing through the anodes m and 210 and between the deflecting plates 2 and impinging on the comb M2 is indicated by dotted lines 218.

A sheet or tape 220 of appropriately sensitized material such as paper is placed immediately adjacent to or in contact with the outer ends of the comb conductors 2l2 and is backed by a knife-edge anode Hi. The latter is connected through ground'to the positive terminal of the battery 2 whose negative terminal is connected to the cathode 206. Thus the electron current of the cathode beam, unless it be reduced in strength by action of the modulating grid 201, passes through the particular comb conductor onto which it is instantaneously directed, through that part of the paper .220 adjacent the outer end of the comb conductor and to the anode 22l, leaving on the paper a local alteration in its light characteristics by discoloration or otherwise.

The sensitive paper tape 220 and the manner inwhich it is affected by the current emerging from the comb wires may be of any suitable type. For example, the paper may be electrolytically sensitive, being moistened to render it active and conductive, or it may be a carbon-loaded paper with a thermally sensitive surface in direct contact with the comb wires, the carbon-loaded paper itself being in contact with the knife edge in which case the mark is made by the heat generated when the current passes through the sensitive surface by conduction. Still other types or forms of sensitive paper and methods of forming impressions on them will occur to those skilled in the art as appropriate for use in connection with the invention.

Current passing from the comb conductors 212 starting point. In order that this starting point to the anode 22l through the paper 220 produces a voltage drop across the paper'due'to its electrical resistance and holds the comb conductors below the anode voltage by the amount of this voltage drop. For best operation, the conductive lining 213 should be maintained at a potential not far from that of the comb. For a particular set of conditions this may be secured by the use of a battery connected between the anode 211 and the conductive lining, but it has been found preferable to employ, instead, the voltage drop across a resistor'2l6 which is connected from the anodeto the conductive lining and the accelerating electrode 210 due to a part of the emitted electron current which impinges on the latter. This connection is believed to serve the additional purpose of compensating for variations in the resistance of the sensitive paper. A tentatively offered explanation of this efiect is as follows. When, for example, paper of higher resistance than normal is employed, the combconductcrs 2l2 tendto fall in potential. Secondary electrons knocked out of the comb by the m pact of the primary electrons of the beam 215 migrate to the conductve lining 213 and in effect reduce the useful current passing through the paper. At the same time this secondary current, by flowing through the resistor 2H5, increases the voltage drop across the latter in a sense to restore the potential difference between.

the comb H2 and the lining 2&3 to its original value.

It has been found that with a beam tube of the type described in Patent 2,273,793 and adjusted to deliver a beam current of the order of' 500 microamperes, good results are secured when the resistor 216 is selected to provide a voltage drop of the order of 20 volts with ordinary commercial sensitive papers.

If sufiiciently high voltage is available, direct.

conduction is unnecessary since an arc can be 1 caused to pass through ordinary dry white paper in contact with carbon or dyed wax paper. The heat of the arc may then cause a deposit of carbon or dye on the white paper,

Whatever the means employed for forming an impression on the tape, it is of course not limited to the formation of a single record. With'a sufficiently high voltage between comb wires and knife-edge anode, a plurality of copies may be made simultaneously.

Any 'suitable means may be employed for defleeting the cathode beam along the comb in the In operation the condenser 224 is charged by the battery 222 through the resistor 223 and the resulting condenser voltage is impressed between the deflecting plates 21 I. As the condenser voltage increases the cathode beam 2l5 is deflected along the row of comb conductors 2l2. After a stipulated time has elapsed, the condenser 224 is short-circuited'by closure of one of the switches 225, 226 and the cathode beam flies back to its 7.5

may be at one end of the comb instead of at its center, the battery 222 may be provided with an adjustable ground 221. If desired, a constant current resistor, for. example, a pentode tube, may be employed as the charging resistor, but when a sufficiently high battery voltage isreadily available excellent results can be secured without resorting to this expedient.

The sensitized paper tape 220 is arranged to be intermittently advanced by rotation of a driving roller 230 while being held in correct position by an idler roller 23!. The driving roller 23a is arranged to be driven through worm gears 232, 233 and ratchet wheel 234, operable by two pawls235, 236 and which in turn are actuated by two relays 231, 238 in a manner similar to that shown in Fig. 1 and described above. It may likewise be provided with a free-wheeling device and a crank for rapid manual device, just as in Fig, 1. As in the case of the transmitter, one of the pawls 235 is arranged to advance the ratchet wheel 234 by the width of one tooth against the tension of its spring 239 and therefore the sensitized paper tape 22!] by the width of a single scanning line when its relay 238 is energized. Likewise, energization of the double line advance relay 238 advances the other pawl 236 against the tension of its spring 240 by a distance of two ratchet wheel teeth and the sensitized paper 220 by the width 'of two scanning lines.

In each case the motion of the pawl may be adjusted and limited by proper positioning of stops 249, M2.

A contact switch 225 is coupled to the pawl 235 and arranged to be closed when the relay 231 which actuates that pawl is energized. Likewise another switch contact 226 is coupled to the pawl 236 and arranged to be closed when the relay 238 is energized. These switches are connected in parallel across the condenser 22% so that the latter is short-circuited each time that either one of the two pawls 2'35, 236 is operated.

The second path from the input terminals 206 leads through a filter 250 and a rectifier 25! to the single line advance relay 2,31 and the third path likewise leads through a filter 255 and a rectifier 256 to the double line advance relay 238. The filters 250 and 255 may appropriately be selected to pass single line advance signals and double line advance signals, respectively. For

example, the pass-band of the single line advance filter 250 may extend. from 0 to 600 cycles to include the 500-cycle signals of the generator 93 and exclude the l000-cycle signals of the generator 90, while,the pass-band of the double line advance filter 255 may extend from 700 to 1200 cycles to include the signals of the generator but exclude those of the generator 93 and also those of the image signal carrier generator 55. The rectifiers are included to prevent the chatter which usually accompanies operation of relays on alternating current alone. A condenser 253,

251 ispreferably connected across each of the deflection of the beam 2|! along the comb M2. The resistor 223 is preferably of the variable type so that this speed may be regulated. It may be set at such a value that the beam 2 l5 travels the full length of the comb M2 in the time occupied by the scanning of one line at the transmitter independent of the length of the copy line and of the transmitter scanning rate. Since the flyback of the beam 2|! takes place practically instantaneously, the adjustment is preferably such that the beam 2l5 continues to be-defiected beyond the end of the comb 2l2 during the return sweep of the scanning mirror 42 at the transmit- Throughout the operative portion of the ter. sweep of the electron beam along the comb, the strength of this beam is modulated by the signals impressed on the grid 201, with the result that more or less current passes through the successive comb conductors, through the sensitized paper to the knife edge anode 22l to leave impressions of more or less darkness on the paper 220 and so reconstitute a replica oi the original copy i.

At the conclusion of each scanning line, either the 500-cycle single line advance signal or the 1000-cycle double line advance signal arrives at the input terminals 200 of the apparatus. In the one case the sensitized paper 220 is advanced by the relay 23! by the width of a single scanning line and in the other case the paper is advanced by the relay 238 by the width of two scanning lines. In either case, one or other of the switches 225, 226 is closed to short-circuit the condenser m and return the electron beam M5 to its starting point in preparation for the scanshown. For example if the well-known relaxa-' tion oscillator be employed, it may be arranged to be tripped electrically on the arrival of either of the single line advance or double line advance signals.

Other receiver apparatus appropriate for use "in connection with the invention will occur to those skilled in the art. For example, a receiver apparatus of the well-known rotating drum type .may be employed, in which single line advance is ordinarily eifected by a continuous motion of the printer along a line parallel with the axis of the drum. In this case double line advance signals would be used to operate any suitable mechanism for skipping a single line. I

In the embodiments above described the advance aperture is spaced from the main aperture by the width of a single line and the apparatus is so arranged that the two scanning spots travel side by side the full length of two adjacent scanning lines, with the result that when the advance aperture sees only white areas throughout the length of its full line, that line is skipped in the course of the next copy advance. It will be apparent that a difierent spacing of the scanning vance is likewise entirely feasible. For example, the apparatus may easily be modified to skip two lines instead of one and to do so when no dark picture areas appear anywhere within the compass of two successive lines. This may be simply accomplished merely by doubling the width, in a direction perpendicular to the scanning direction, of the advance aperture. If this should result in too great a reduction in sensitivity, it may likewise be accomplished without an increase in the size of the advance aperture by the use of a modification in the trigger circuit. For

example, the trigger device may be modified to recall any dark elements seen in the course of two successive scanning lines merely by providing that the switch which deenergizes it shall be momentarily closed only once for every other revolution of the main scanning shaft by the spots or apertures is entirely feasible and an associated different choice of the ensuing copy adprovision of a cam which rotates at one-half the speed of its shaft.

The apparatus may likewise be modified to effect a skippingof a portion' of a scanning line instead of a full scanning line and to effect this skipping when no dark areas appear anywhere in the particular portion in question. Furthermore, it may easily be adapted by modifications of a minor nature to transmission of negative" copy, that is, copy consisting of light marks on a dark background.

Modifications to suit the exigencies of scanning systems other than the one above described will occur to those skilled in the art. For example, in apparatus in which scanning is accom plished by continuous rotation of a cylinder about which the copy is wrapped, normal single line advance is accomplished continuously, as for example by the advance. of a screw thread. In this case, signals from a premonitory aperture may be supplied to an advancing mechanism to effect intermittent axial advance of the cylinder as a whole with respect to the scanning aperture, when in the course of a full revolution of the cylinder, the advance aperture has seen no dark areas.

Various modifications of the optical apparatus above described will likewise occur 'to those skilled in the art. In particular, if desired, more than two scanning apertures may be employed, either in accordance with the embodiment of Fig. 1 or with that of Fig. 2. In the latter case a'three-way color separation, for example by pairs of red, yellow and blue filters, may be employed to confine the light of each beam entirely in the path to itsassociated photoelectric cell.

The apparatus described above is of a type particularly suited to transmission of image signals from opaque copy, for example printed on typewritten matter, and the reconstitution thereof. As stated 'at the outset, however, this is by way of example only and in no sense restrictive, since modifications may occur to those skilled in the art which adapt the apparatus to use with fields of other types, for example a transparency, either positive or negative, or indeed, a bare optical image of an object field.

In the case of each 01 the modifications here alluded to, corresponding modifications of the receiver apparatus may occur to those skilled in the-art.

What is claimed is:

1. In an image signal transmission system, means for scanning an elemental line of a field to be transmitted and deriving image signals therefrom. means for scanning another like eleaasz-na said first-named elemental areas and deriving auxiliary signals therefrom, and means controlled by said auxiliary signals for causing said firstnamed scanning means to skip blank areas scanned by said second-namedjscanning means.

3. In an image signal transmission system, a scanning aperture, means including said aperture for successively scanning elemental areas of .a line of a field to be transmitted, means for advancing said field by a single line width, means for advancing said field by a double line width, and means for selectively actuating said fieldadvancing means in dependence on the light characteristics of elemental field areas seen by said aperture.

4. In an image signal, transmission system, means for producing image signals related to information-containing areas of a field to be transmitted, means for producing auxiliary signals in response to blank areas-ofv said field, and means for causing said image signal producing means to skip blank areas of said field under control of said auxiliary signals.

5. In an image signal transmission system, means for illuminating a plurality of elemental areas of a field to be transmitted, said elemental areas being spaced apart by at least the width oi? a scanning line, means for sweeping said illuminated areas over said field to scan said field, a plurality of separate photoelectric devices, means for projecting light from each one of said areas onto an associated one of said photoelectric devices, means for deriving image signals from one of said devices, means for deriving auxiliary signals from another of said devices, and means for advancing said field alternatively by oneline width or by more than one line width means for applying said signals to said trigger device to provide an energizing impulse when said aperture sees an elemental field area in any position in said field and diflering in light characteristic from its background, and means for pro- I ducing relative movement of said field and said scanning means under control of said energy.

8. In an image signal transmission system, a scanning aperture, means including said aperture for successively scanning elemental areas of a line of a field to be transmitted to derive signals, a trigger device associated with said scanning means arranged to maintain a first state prior to energization of said device by an impulse and to trip to a second state upon energization by an impulse and maintain said second state vthenceforward until disabled, said trigger device being arranged to deliver energy in' relation to its state, means for applying said signals to said trigger device to provide an energizing impulse when said aperture sees an elemental field area in any position in said field and diifering in light characteristic from its background, means for producing relative movement of said field and said scanning means under control of said energy, and meansior disabling said trigger device at the conclusion of each line scan.

9. In an image signal transmission system, a

scanning aperture, means including said aperture for scanning elemental areas of a line of a field to be transmitted in succession to derive a signal having one characteristic when said aperture has seen one or more dark elemental areas in a scanning line and another characteristic when said aperture has seen no dark areas in said line, a generator of oscillations of a first frequency,

including said source, said aperture and said element for scanning elemental areas of a field to derive image signals, an auxiliary light source, an auxiliary aperture and an auxiliary light responsive element, means including said source,

, ment.

'7.'In an image signal transmission system, a scanning aperture, means including said aperture for scanning elemental areas'of a field to be transmitted in succession to derivesignals, a

trigger device associated with said scanning means arranged to maintain a first state prior to energiztion of said device by an impulse and to trip to a second state upon energization by an impulse, said trigger device beingarranged to deliver energy in accordanced with its Stat il agenerator of oscillations of a second frequency, means responsive to oscillations of said first frequency for advancing said field by a single line width, means responsive to oscillations of said for projecting beams of light of said sources and sensitive .to light of the color illuminating said area and insensitive to light of the color illuminating others of said areas, means for deriving image signals from one of said devices, meansv for deriving auxiliary signals from the other of said devices, and means for varying the advance of said scanning means relatively to said field under control of said auxiliary signals.

11. In a system for deriving and transmitting image signals from a field scanned, a principal aperture, an auxiliary aperture, means for scanning an elemental line of said field by said principal aperture and deriving image signals therefrom, means for scanning another elemental field line in advance ofsaid first-named line by said auxiliary aperture and deriving auxiliary signals therefrom, and means for advancing said field relatively to both of said'scanning means under control of said auxiliary signals and by distances related to the tone values of areas seen by said auxiliary aperture.

12. In a system tor deriving and transmitting image .signals from a field scanned, a principal aperture, an auxiliary aperture, means for scanning elemental areas of a field line in succession by said principal aperture, means for scanning other elemental areas in advance of said firstnamed areas in a like succession by said auxiliary aperture, means fortranslatlng tone values of areas seen by said principal aperture into image signals, and means for deriving single-line-advance signals from non-uniform areas seen by said auxiliary aperture and plural-line advance signals from unii'orm areas seen by said auxiliary aperture.

13. In a picture transmission system, 'means for scanning an elemental line of a field to be transmitted and deriving image signals therefrom, means for scanning another elemental field line in advance of said first-named line and deriving auxiliary signals therefrom, means for storing said auxiliary signals until the termination of a line scan, and means for advancing said fieldat the termination of said line scan under control of said stored signals.

14. In a picture transmission system, means for scanning. successive elemental areas of a field to be transmitted and deriving image signals therefrom, means for scanning other like elementel areas of said field in advance of said firstnamed areas, and deriving auxiliary signals therefrom, means for storing said auxiliary sigrials until the termination of a stipulated period, means for advancingf said field at the termination of said period under control of said stored signals, and means for reconditioning said storage means to repeat said signal storage.

15. In the art or picture transmission, the method which comprises scanning successive elemental areas of a line of a field to be transmitted and deriving image signals therefrom, simultaneously scanning ucoessive elemental areasoi another'line oi d field in advance or said first- .narned line 'and deriving auxiliary signals therefrom, storing said auxiliary signals derived in the 7 course of scanning a line until the termination areas, means for scanning other like areas in advance of said first-namedareas'to produce auxiliary signals related to blank areas positioned at random in said field, means for transmitting said image signals and said auxiliary signals to a receiver station, and at said receiver station, an impression-receiving medium. means for reconstituting information-containing areas on said medium under control of said image signals, and means for skipping certain areas of said medium corresponding to blank areas of said field under control of said auxiliary signals.

17. In combination with image s'ignaltransmitter apparatus including scanning means and v delivering image signals related to elemental areas scanned and auxiliary signals related to receiver apparatus which comprises an impression receiving medium, a scanning element arranged to scan said medium in one direction and form impressions thereon, means for modulating the action 01 said'scanning element to vary said impressions under control or received image signals, means for synchronizing the movement of said scanning element with the movement of the transmitter scanning means under control of said auxiliary signals, and means for advancing said medium relatively to said scanning element by various distances in a direction perpendicular to said scanning direction under control of correspondingly various components of said auxiliary signals.

18; Means for scanning a field containing parallel 'rows of markings with interspersed'blank strips between said rows to set up image current, said means comprising means for scanning along parallel elemental paths extending in the direction of sald'rows, advance scanning means for testing in advance of said-first scanning means along a path extending in the same direction and .lor setting up a distinctive signal when said tested path is along a blank strip, and means responsive to said signal for determining the selection or the next elemental path taken by said first scanning means. v

19. In an image signal transmission system. means for scanning an object field in parallel transverse elemental lines to produce image signals related to information-containing areas of said field, means ior producing auxiliary si nals in response to blank areas of said field, and means under control of said auxiliary signals for causing said image-signal-producing means to skip some at least of any blank elemental lines present in the field which is scanned.

EDMOND BRUCE. WILLIAM HERRIO'I'I'. 

